event: various builder changes
event: various builder changes

- Add At method to set time.

- Factor out common log code.

- Fix minor bugs.

Change-Id: I98d6ff262e3178b3470945bd3644dc9474dc3d56
Reviewed-on: https://go-review.googlesource.com/c/exp/+/319729
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/exp,golang/exp,golang/exp